[Sass] Have Sass::Script::String distinguish between strings and identifiers.

lib/sass/script/functions.rb

@@ -170,6 +170,8 @@ module Sass::Script
       #     assert_type value, :Number
       #
       # Valid types are `:Bool`, `:Color`, `:Number`, and `:String`.
+      # Note that `:String` will match both double-quoted strings
+      # and unquoted identifiers.
       #
       # @param value [Sass::Script::Literal] A SassScript value
       # @param type [Symbol] The name of the type the value is expected to be

lib/sass/script/lexer.rb

@@ -199,7 +199,7 @@ module Sass
       def string(re, open)
         return unless scan(STRING_REGULAR_EXPRESSIONS[[re, open]])
         @interpolation_stack << re if @scanner[2].empty? # Started an interpolated section
-        [:string, Script::String.new(@scanner[1].gsub(/\\([^0-9a-f])/, '\1').gsub(/\\([0-9a-f]{1,4})/, "\\\\\\1"))]
+        [:string, Script::String.new(@scanner[1].gsub(/\\([^0-9a-f])/, '\1').gsub(/\\([0-9a-f]{1,4})/, "\\\\\\1"), :string)]
       end
 
       def number

lib/sass/script/parser.rb

@@ -221,7 +221,7 @@ On line #{name.line}, character #{name.offset}#{" of '#{filename}'" if filename}
 Implicit strings have been deprecated and will be removed in version 3.0.
 '#{name.value}' was not quoted. Please add double quotes (e.g. "#{name.value}").
 END
-          node(Script::String.new(name.value))
+          node(Script::String.new(name.value, :identifier))
         else
           args = arglist || []
           assert_tok(:rparen)

lib/sass/script/string.rb

@@ -1,7 +1,7 @@
 require 'sass/script/literal'
 
 module Sass::Script
-  # A SassScript object representing a string of text.
+  # A SassScript object representing a CSS string *or* a CSS identifier.
   class String < Literal
     # The Ruby value of the string.
     #
@@ -9,8 +9,26 @@ module Sass::Script
     attr_reader :value
     alias_method :to_s, :value
 
+    # Whether this is a CSS string or a CSS identifier.
+    # The difference is that strings are written with double-quotes,
+    # while identifiers aren't.
+    #
+    # @return [Symbol] `:string` or `:identifier`
+    attr_reader :type
+
+    # Creates a new string.
+    #
+    # @param value [String] See \{#value}
+    # @param type [Symbol] See \{#type}
+    def initialize(value, type = :identifier)
+      super(value)
+      @type = type
+    end
+
     # @see Node#to_sass
     def to_sass
+      return self.value if type == :identifier
+
       # Replace single backslashes with double. Really.
       value = self.value.gsub("\\", "\\\\\\\\")
       return "\"#{value}\"" unless value.include?('"')